In the automotive and related coatings industries, it is estimated that between 20 to 40% of the total volume of paint sprayed is overspray. In the application of organic paints such as epoxy resins, high solids enamel paints and the like, it is necessary to trap the oversprayed paint. In large industrial applications such as auto body coating, this is accomplished by the use of water curtains on the interior booth walls, whereby the oversprayed coating material is incorporated into the water as it cascades down the walls of the booth into a reservoir. Typically, this water is recirculated back to the booth by means of recirculating pumps. The agglomeration and accumulation of live coating material in the water of the paint spray booth results in serious problems such as blockage of the pipes and circulating pumps as well as an accumulation of paint on the walls of the paint spray booth behind the water curtain. As more and more coating material is sprayed in the booth, the overspray material removed from the air builds up in the water in the form of tar-like coherent sludge which in a short time can foul the pumps and lines which circulate the booth's water. Furthermore, this sludge is extremely difficult to remove from the pump, lines, reservoir, and other internal surfaces of the system with which it comes in contact. The accumulation of raw paint masses in the water reservoir also creates serious maintenance problems when the system is periodically cleaned out, requiring much effort to remove the heavy build-up of coating material from the reservoir.
It is therefore desirable, to treat the water in the booth in such a way as to render the oversprayed coating material free of stickiness and tackiness so that it readily separates from the water, does not adhere to the spray booth walls, pipes, pumps; or other surfaces and internal components of the spray booth system, and maintains a floating, detackified condition.
Various treatments have been reported in the patent literature. For example, U.S. Pat. No. 3,861.887 discloses the treatment of paint booth wash water with a blend of polycationic water dispersible polymer with a water-soluble salt of an amphoteric metal to reduce the tackiness of paint. One problem with this approach is that the use of amphoteric metals such as zinc can create additional disposal concerns regarding the wastewater and sludge.
Another approach has been to use clay-based treatment. In U.S. Pat. No. 4,504,395, it is disclosed that certain hectorite clays can be effectively used to detackify oversprayed paint. One problem with this approach is that the resultant sludge produced by the clays is difficult to dewater, resulting in larger volumes of potentially hazardous materials requiring disposal in a secure landfill.
Yet another approach is to use a melamine-aldehyde acid colloid solution as disclosed in U.S. Pat. No. 4,656,059. The melamine-aldehyde acid colloid solution is produced by causing melamine to react with aldehyde, preferably formaldehyde or paraformaldehyde, to form a melamine-formaldehyde reaction product and then adding acid to the melamine-formaldehyde reaction product to form the melamine-aldehyde acid colloid solution. The use of this composition entails the following disadvantages: (a) the paint is not completely detackified, (b) the melamine-aldehyde acid colloid product typically contains greater than 0.1% free formaldehyde which is a suspect carcinogen, and (c) as a consequence of incomplete detackification the paint cannot be effectively dewatered and thus results in increased hazardous waste sludge production.